An energy storage device, especially a lithium secondary battery, has been widely used recently for a power source of a small-sized electronic device, such as a mobile telephone, a notebook personal computer, etc., and a power source for an electric vehicle or electric power storage. Since there is a possibility that such an electronic device or a vehicle is used in a broad temperature range, such as a high temperature in midsummer, a low temperature in an extremely cold region, etc., it is demanded to improve electrochemical characteristics with a good balance in a broad temperature range.
In particular, in order to prevent the global warming, it is an urgent need to reduce the CO2 emission. Among eco-friendly vehicles mounted with an energy storage apparatus composed of an energy storage device, such as lithium secondary batteries, capacitors, etc., early dissemination of a hybrid electric vehicle (HEV), a plug-in hybrid electric vehicle (PHEV), or a battery electric vehicle (BEV) is demanded.
Since a vehicle is long in moving distance, there is a possibility that the vehicle is used in regions in a broad temperature range of from a very hot region of the torrid zone to an extremely cold region. In consequence, in particular, these onboard energy storage devices are required such that even when used in a broad temperature range of from high temperatures to low temperatures, the electrochemical characteristics are not worsened.
In the present specification, the term, lithium secondary battery, is used as a concept also including a so-called lithium ion secondary battery.
A lithium secondary battery is mainly constituted of a positive electrode and a negative electrode, each containing a material capable of absorbing and releasing lithium, and a nonaqueous electrolytic solution containing a lithium salt and a nonaqueous solvent, and a carbonate, such as ethylene carbonate (EC), propylene carbonate (PC), etc., is used as the nonaqueous solvent.
In addition, metallic lithium, a metal compound capable of absorbing and releasing lithium (e.g., a metal elemental substance, a metal oxide, an alloy with lithium, etc.), and a carbon material are known as the negative electrode. In particular, a lithium secondary battery using a carbon material capable of absorbing and releasing lithium, such as coke, artificial graphite, natural graphite, etc., is widely put into practical use.
For example, in a lithium secondary battery using, as the negative electrode material, a highly crystallized carbon material, such as natural graphite, artificial graphite, etc., it is known that a decomposition product or gas generated by reductive decomposition of a solvent in a nonaqueous electrolytic solution on the negative electrode surface on charging hinders a desired electrochemical reaction of the battery, so that worsening of cycle properties is possibly caused. When decomposition products of the nonaqueous solvent are accumulated, absorption and release of lithium on the negative electrode may not be performed smoothly, and the electrochemical characteristics when used in a broad temperature range are liable to be worsened.
Furthermore, it is known that a lithium secondary battery using a lithium metal or an alloy thereof, or a metal elemental substance, such as tin, silicon, etc., or an oxide thereof as the negative electrode material may have a high initial battery capacity, but the battery performance thereof, such as battery capacity and cycle properties, may be largely worsened because the micronized powdering may be promoted during cycles, thereby bringing about accelerated reductive decomposition of the nonaqueous solvent, as compared with the negative electrode formed of a carbon material. When such a negative material is micronized, or decomposition products of the nonaqueous solvent are accumulated, absorption and release of lithium on the negative electrode may not be performed smoothly, and the electrochemical characteristics when used in a broad temperature range are liable to be worsened.
Meanwhile, it is known that in a lithium secondary battery using, as a positive electrode material, for example, LiCoO2, LiMn2O4, LiNiO2, LiFePO4, etc., on an interface between the positive electrode material and the nonaqueous electrolytic solution in such a state that the nonaqueous solvent in the nonaqueous electrolytic solution is charged, a decomposition product or gas generated by a partial oxidative decomposition which is caused locally hinders a desired electrochemical reaction of the battery, and therefore, the electrochemical characteristics when used in a broad temperature range are liable to be worsened, too.
In the light of the above, the battery performance was worsened due to the matter that the movement of a lithium ion is hindered or the battery is expanded by a decomposition product or gas when the nonaqueous electrolytic solution is decomposed on the positive electrode or negative electrode. Irrespective of such a situation, the multifunctionality of electronic devices on which lithium secondary batteries are mounted is more and more advanced, and power consumption tends to increase. For that reason, the capacity of lithium secondary battery is thus being much increased, and the space volume for the nonaqueous electrolytic solution in the battery is decreased by increasing the density of the electrode, or reducing the useless space volume in the battery, or the like. In consequence, it is a situation that the electrochemical characteristics when used in a broad temperature range are liable to be worsened due to even a bit of decomposition of the nonaqueous electrolytic solution.
PTL 1 proposes a nonaqueous electrolyte secondary battery containing a sulfuric acid ester compound, such as dimethyl sulfate, etc., for the purpose of increasing cycle properties and suggests that in view of the fact that such a compound reacts on the negative electrode surface to form a surface film, the cycle properties are improved.
PTL 2 proposes an electrochemical battery containing a sulfuric acid ester compound, such as methyl sulfate, etc., for the purpose of improving a first cycle irreversible capacity and keeping a low-temperature cycle capacity and suggests that in view of the fact that such a compound reacts on the negative electrode surface to form a surface film, the first cycle irreversible capacity is improved.
PTL 3 proposes a secondary battery containing, as an anionic active agent, an alkyl sulfate, such as lithium dodecylsulfate, etc., for the purpose of improving high-rate discharging properties and suggests that by improving wettability of the negative electrode, the high-rate discharging properties are improved.
PTL 4 proposes a secondary battery containing a cyclic disulfonic acid ester and a surfactant for the purpose of improving a long-term cycle life and suggests that wettability of an electrolytic solution to a negative electrode is improved so that the cyclic disulfonic acid ester penetrates into details, whereby SEI is uniformly formed and the cycle properties are improved.
In general, as an electrolyte salt which is used for energy storage devices, such as lithium secondary batteries, etc., an electrolyte having sufficient solubility in an organic solvent, for example, LiPF6, LiBF4, LiClO4, LiN(SO2CF3)2, LiN(SO2C2F5)2, LiCF3SO3, etc., is selected, and of those, especially, LiPF6 is widely used (PTLs 5 and 6, etc.).
PTL 7 discloses a nonaqueous electrolytic solution containing LiPF6 as a main lithium salt, to which a small amount of a perfluoroalkane sulfonic acid salt, such as lithium perfluorobutanesulfonate (LiPFBS), etc. and indicates that not only the charging and discharging cycle properties may be improved, but also the time for injecting the electrolytic solution into a battery may be shortened.